1. Field of the Invention
The present invention relates to a diffractive optical element and a polarization converting element using the diffractive optical element. The diffractive optical element is used under a plurality of wavelengths, or a plurality of band light. The diffractive optical element and polarization converting element are suitably employed in various sorts of optical appliances, for instance, an imaging optical system, a projection optical system (projector), an image processing apparatus, and a semiconductor manufacturing apparatus.
2. Related Background Art
Conventionally, diffractive optical elements may be used as diffractive lenses having purposes of reducing chromatic aberration, which is described in, for example, SPIE Vol. 1354 International Lens Design Conference (1990).
Also, diffractive optical elements may be used as color separation grating having purposes of performing color separations by utilizing different diffraction angles with respect to each of wavelengths, which is described in, for example, Japanese Patent Publication No. 5-46139.
Very recently, another diffractive optical element called as an SWS grating (Sub-wavelength structured grating) having a minute periodic structure, in which a grating period of a diffractive optical element is smaller than a used wavelength. A specific attention is paid to these SWS gratings which are disclosed in, for instance, Japanese publication “KOGAKU”, volume 27, No. 1, published in 1998, on pages 12 to 17.
As to this SWS grating, it is known that such SWS gratings may own various functions such as a double refraction (birefringence) wavelength plate, an antireflection structure, and a polarization beam splitter, depending upon grating structures thereof. Then, as to these functions, various reports have been made in which there is a small optical performance variation caused by changes in incident angles of light beams entered into this SWS grating, and the SWS grating may have optically superior features.
Among these SWS gratings, as a diffractive optical element having a function of a polarization beam splitter, such a structure shown in FIG. 8 is disclosed in the publication “O plus E” Vol. 21, No. 136 (March in 1991), on pages 86 to 90, and also such a structure indicated in FIG. 9 is disclosed in the publication “O plus E” No. 12 (December in 1999), on pages 1554 to 1559.
In the diffractive optical element of FIG. 8, a portion of a grating period “Pt” is constituted by an SWS grating 5. The SWS grating 5 corresponds to a rectangular grating constituted by a material “n1” and a material “n2” of an element boundary. A grating period of a minute periodic structure is “p1”, and an occupation ratio of the material “n2” occupied within an 1 period “P1” (namely, filling factor) is equal to “f1”. Then, the SWS grating has a thickness “d1” on the side of the material n2, and a thickness “d2” on the side of the material n1.
Similarly, in the diffractive optical element of FIG. 9, a portion of a grating period Pt is constituted by the SWS grating 5. In this structure, a portion of a multiple layer film made of a material “n1” and another material “n2” is such an SWS grating constituted by a triangular grating, and is made in contact with another material “n3” at an element boundary.
In any of the diffractive optical elements shown in FIG. 8 and FIG. 9, an S-polarized light component is propagated as zero-order diffractive light along one direction, whereas a P-polarized light component is propagated in such a manner that this P-polarized light component is separated into two directions of (+) first-order diffraction light and (−) first-order diffraction light. This implies that the light amounts of emitted light beames are made approximately two times different from each other, depending upon the polarization directions. When these diffractive optical elements are applied to various fields as the polarization separation elements, the above-described propagation characteristics are not preferable.
Next, a polarization beam splitter 100 shown in FIG. 10 is well known in this field as a polarization separation element using a thin film. In this polarization separation element, a thin film is formed on a boundary surface 102 on which two sets of triangular cylinder prisms 101 are joined to each other, and incident light is caused to pass through the boundary surface, or to be reflected on this boundary surface, depending upon a polarization direction thereof, so that the polarized-light (beam) is separated.
In the case that each of the polarized light beams is entered at the designed incident angle, this incident polarized light beam may pass through the boundary surface, or may be reflected thereon with a transmittance or reflectance of alomost 100%. However, these polarization separation elements own such a drawback that when the incident angle thereof is shifted from the designed angle value by several degrees, the resultant polarization separation characteristic is considerably deteriorated.
Also, very recently, as a functional element to which a polarization separation element of a thin film is applied, such a polarization converting element 103 shown in FIG. 11 has been proposed in, for example, Japanese Laid-open Patent Application No. 10-39136.
When the structure is explained, a light beam “La” having random polarization directions, which is entered from an opening portion A1 provided on a light shielding member 6, is separated into a P-polarized light component and an S-polarized light component by a polarization separation thin-film 102.
The P-polarized light component passes through the polarization separation film 102, and thereafter the polarization direction of this P-polarized light component is converted into S-polarized light by a ½-wavelength plate 8 provided on the exit side, and then the S-polarized light component emerges therefrom.
On the other hand, after S-polarized light component is reflected by the polarization separation thin-film 102, the reflected S-polarized light component is again reflected by a reflection mirror 105, and then, the reflected polarized light maintaining the S-polarization emerges therefrom.
As a result, when the light beam La having the random polarization directions is entered into the polarization converting element 103, the light beam the polarization state of which is aligned as S-polarized light emerges therefrom.
This polarization converting element 103 is suitably employed so as to effectively illuminate such an element having a polarization characteristic as liquid crystal, while reducing a loss of a light amount.
However, this polarization converting element is manufactured as follows: That is, the polarization separation thin film 102 and the reflection mirror 105 are alternately overlapped with each other to be joined to the flat plate glass 104. The resultant element member is diagonally cut away, and the cut surfaces thereof are polished, and thereafter, the ½-wavelength plate 8 is adhered to a portion of this polished element member so as to manufacture the polarization converting element 103.
This manufacturing method requires very large numbers of manufacturing steps, and the very complex element is finally manufactured.
In the diffractive optical elements using the SWS grating in the conventional prior art mentioned above, since the light beam of the specific polarization direction is separated into the two different directions due to the structure thereof, there is a problem that the use efficiency of the light beam is lowered. On the other hand, the polarization separation element having the thin film has the following problems. That is, the incident angle characteristic of this polarization separation element is highly sensitive, and/or the manufacturing method of this polarization separation element is very complex.
The present invention has an object to provide a diffractive optical element and a polarization converting element with employment of such a diffractive optical element. That is, the diffractive optical element may be used in a similar manner to a polarization separation element having a thin-film structure, and while since a structure of an SWS grating is set in a proper manner, diffraction corresponding to each of polarization directions is effected only in a specific order.
Another object of the present invention is to provide a diffractive optical element and a polarization converting element with employment of this diffractive optical element, which has an SWS grating structure manufactured under better condition, and is capable of separating polarized light.